1. Field of the Invention
The present invention relates to a hydraulic drive and, more particularly, to a hydraulic drive for a hydraulic press. The invention moreover relates to a method for operating such a drive.
2. Description of the Related Art
Hydraulic drives are well known from the current state of the art. In practice it is desirable for hydraulic drives, in particular for hydraulic drives for hydraulic presses, to provide a hydraulic drive that on the one hand provides a rapid movement of a drive piston with low force in a so-called rapid stroke or rapid movement, and with which on the other hand a slower action with great force is possible in a so-called load stroke or load movement.
Various drives are known for this purpose from the current state of the art. In one drive with a so-called throttle control, a pump is driven by a motor at constant speed. The control and changeover between rapid stroke and load stroke through control of the volume flow occurs hereby via flow resistances, for example valves. A disadvantage of such a drive with throttle control is the low efficiency due to the occurring flow losses.
Drives having a so-called displacement control system are moreover known from the current state of the art. A drive of this type may for example comprise a variable speed motor that drives two pumps having opposite delivering directions. The two pumps are connected via a hydraulic cylinder in such a way that the pump takes in hydraulic oil from one piston chamber of a hydraulic cylinder, whereas it moves hydraulic oil into the other piston chamber. The changeover from rapid stroke to load stroke, or respectively the speed control of the hydraulic drive occurs through changing of the displacement volume of the pump or respectively through the change in speed of the motor. A disadvantage of such a drive with the displacement control system is that the motor must have a higher speed for the high speed in the rapid stroke, whereas a high maximum torque is required for the high force in the load stroke mode. When using a fixed displacement pump, the motor must be designed accordingly because of this high so-called peak performance and therefore it becomes large, heavy, slow and expensive.
What is needed in the art is a hydraulic drive that avoids efficiency losses and whereby the motor should be able to be produced cost effectively.